Iso-1-cytochrome c and iso-2-cytochrome c from the yeast Saccharomyces cerevisiae are two of the few proteins of known primary structure from a microorganism which is particularly suitable for experimental genetic studies. The isolation of appropriate mutants have been facilitated by enrichment procedures for both forward and reverse mutations. A series of deletions are available, making it possible to conveniently map point mutants and to estimate their positions relative to the iso-1-cytochrome c sequence. The large number of mutants that have been characterized and the selection procedures permit an unprecedented degree of genetic manipulation of nucleotide sequences by recombination. The sequencing of a portion of the gene from frameshift mutations has made it feasible to directly analyze the mRNA of iso-1-cytochrome c with a synthetic oligonucleotide. This synthetic oligonucleotide has been used as a probe for identifying bacteriophage lambda and E. coli plasmids containing segments of yeast DNA corresponding to the cycl gene. Thus the body of information concerning the iso-1-cytochrome c gene, the large number of defined mutants and the available genetic and biochemical techniques are without parallel for any other eukaryotic gene. This iso-cytochromes c system is being employed for investigating numerous problems in molecular biology and genetics, including: the isolation and characterization of structural and regulatory mutants for both iso-1 and iso-2-cytochrome c; the identification of amino acids inserted by various nonsense suppressors and other translational suppressors and the determination of the efficiencies of suppression; the determination of the specific action of numerous mutagens; gene conversion studies and the examination of the relationship of recombination frequencies and nucleotide alterations; the characterization of DNAs and mRNAs specifying the two iso-cytochromes c and their measurements in normal and mutant strains; the determination of DNA sequences of mutations that are in the untranslated regions of the CYC1 and CYC7 gene.